HIS 240 - Occlusion Physiology and Earmold Insertion Effects

OCCLUSION PHYSIOLOGY & EARMOLD INSERTION EFFECTS

• For the purposes of this discussion, "occlusion" is operationally defined as the complete or partial blockage of the auricle and/or the external auditory meatus.

• Implicit in this definition are the psychoacoustic and/or physical perceptions resulting from such conditions.


Occluding the outer ear canal causes high frequency hearing loss for air conducted sounds. The audiograms in the next slide show air conduction thresholds obtained with varying degrees of physical occlusion of the external ear canal.



This effect was controlled through varying ear insert vent size to extend from 80% to 100% occlusion. Note: Ranges of occlusion at or above 80% are not uncommon for many hearing instrument fittings. The subject involved in the Chandler study had "normal" hearing (all measured thresholds were at zero when un-occluded).


• High frequencies are affected first, and, regardless of the degree of occlusion, to the greatest extent.

• Note: At or above 95% occlusion the low frequency thresholds (below 1K Hz) shift as well.


• This same effect can also occur with any configuration of hearing.

• Audiogram A in the next slide shows a moderate to severe sensorineural hearing loss.



Audiogram B (in the previous slide) shows the same subject while completely occluded. The result is a mixed loss which significantly reduces the level at which the subject can perceive sound.


This is of considerable consequence in hearing aid fitting because any amount of hearing capacity reduced by the body of the acoustic

coupling, (traditionally called "insertion loss"), must be replaced artificially by the amplifier of the hearing aid.


The artificial replacement of lost sounds (HI amplification) occurs in ear canal spaces which are made significantly smaller by the presence of the acoustic coupler (earmold or HI).


This limitation of space can easily result in higher delivered sound intensities through decreased distance of sound source to the TM (remember Boyle’s law of physics).



The result of combining high frequency

hearing loss (made worse by occlusion), with high frequency emphasis hearing instruments delivered at close proximity to the TM; creates a good potential for high frequency recruitment (frequency specific abnormal growth in the perception of loudness).


Recruitment, in sensorineural hearing loss, begins at threshold. Threshold represents a point of very low perceived loudness. Yet, a shift in magnitude from a sound presented at a threshold level of 70 dB HL to a presentation level of 80 dB HL results in a sound pressure increase of nearly 200%.


This ten decibel (70-80dB HL) difference, requiring almost a 200% increase in sound pressure loudness, is typical of high frequency thresholds in many SN losses.


When there is an attempt to provide HI stimulation, there is so much pressure involved with the application of amplification, that the cochlea is dealing with tremendous amounts of energy. This will (in turn) often yield sudden increases in the patient/client’s received perception of loudness.


Amplification in the high frequency range (located where that group of sounds is routinely described by "normally hearing" people as “piercing” or “tinny”) makes recruitment management in SN hearing loss much more challenging when the

ear is occluded!


Along with insertion loss and the potential for "overdriving" those recruiting ears, excess high frequency input (through amplification) can produce cochlear distortions.


Commonly created cochlear distortions include: • Harmonics• Sub-harmonics • Summation tones • Difference tones


Cochlear Harmonics

A harmonic occurs when a given wavelength divides into shorter wavelengths or higher frequencies.


Cochlear Sub-Harmonics

A sub-harmonic occurs when a wavelength multiplies into longer wavelengths, or lower frequency sounds.


Summation Tones

With the introduction of multiple frequency sounds, the primary frequencies can add to each other producing a summation tone.


Difference Tones

With the introduction of multiple frequency sounds, the primary frequencies can subtract from each other producing a difference tone.


• The introduction of additional input sounds can cause the inner ear to "naturally produce" additional sounds.

• Cochlear distortion products, including harmonics, summation and difference tones, can occur even at low input levels.

• Although cochlear distortions vary depending on input, high intensity inputs produce higher intensity distortion products sub-harmonics are known to occur at input levels of 90 dB HL at frequencies of 2 KHz or more.


The perceived pitch changes created by increased intensity are detailed on the next slide.



The occurrence of cochlear distortions is probably worsened by the cyto-architectural changes involved with the additional presence of SNHL.It is also likely contributing to the reported background noise and distortion complaints by users of hearing instruments.



• Occlusion causes loss of external ear acoustic resonance.

• This removal of the “familiar” resonance pattern represents a fundamental change regarding the emphasis with which acoustic information was previously presented to the auditory system.


A typical external ear acoustic resonance pattern is shown below:


Traditional definitions of the occlusion effect have focused on the increase in bone conducted sound in the low frequency range when the cartilaginous meatus is occluded.


• When placing a block (occluding) the ear canal, the former acoustic resonant pattern of the ear is lost.

• With the high frequency emphasis taken away, the low frequencies, which carry the greatest potential sound power, will be heard internally more easily.


Increased sensitivity to bone conducted stimuli varies considerably from individual to individual, it occurs predominantly in the low frequencies, and has been measured as a BC threshold improvement of up to 30 dB.

Note: A Weber effect may also occur when one ear is occluded.


Increased sensitivity to bone conduction (BC) under occlusion is of particular concern when considering the voice of a hearing instrument wearer.


• The human voice results from vibrations caused by movement of the vocal folds as air rushes past them.

• These vibrations are modified by the resonant cavities of the skull, which are surrounded predominantly by bone.

• Sound can then be transmitted through the bone to the cochlea.


Sound that is transmitted through bone can stimulate cochlear response in one of two ways:

1. compressional bone conduction, where sound passes from the temporal bone through the outer shell of the cochlea.

2. inertial bone conduction, caused when the bony portion of the external auditory meatus transmits sound to the tympanic membrane through the annulus or the air in the canal.


• The loudness of inertial bone conducted sound is increased through occlusion.

• The human voice is capable of producing sound pressures (measured in the throat) of 140 dB SPL (Killion et al, 1988).


Plugging the outer ear canal causes the delivery of high frequencies to be reduced.


• The low frequency (long wavelength sounds), will travel through the bone structures with the least amount of loss; will be delivered to the bony portion of the meatus with the greatest facility.

• This occurs at sound pressure levels that have been measured, under occlusion, at near 100 dB SPL (Killion et al, 1988).



• FIGURE 4 (the previous slide) illustrates some in-situ measurements, and the increase in low frequencies is well defined using the vocalized OO & EE .

• Killion and his associates called the results of this process "self-masking."


• “Self-masking” refers to the fact that the low frequency sounds emphasized by occlusion could cause other signals to be reduced to in-audibility.

• This is the result of low frequency sounds having more acoustic power, and thus masking the higher frequencies.


Examples include not only the user’s own voice, but sounds made while eating, shaving, etc. The introduction of any low frequency sound input can result in the 'upward spread of masking'.


A sound of 500Hz frequency has a wavelength of about two and two-tenths feet. One cycle of such a sound takes up over two feet of space in the atmosphere as it travels forth from its source. Note: The cochlea is about 31mm in total length from base to apex (Zemlin, 1988).


• A low frequency sound wave has wavelengths which are so great that they will excite not just a single area of low frequency responsive hair cells, but also high frequency responsive hair cells.

• Due to the overall areas of pressure, and the introduction of harmonic, or multiple frequency components masking of those high frequencies easily occur.


This spread of masking into the higher frequencies, called the 'upward spread of masking', is probably further exacerbated by the fact that the basilar membrane is narrower at the base of the cochlea, and grows wider at the apex (Zemlin, 1988). Reference next slide.



After review of the previous slide, it is indeed a valid assumption that a narrower membrane will be more easily set into motion than a wider one, causing the high frequency sensory cells at the base of the basilar membrane to also be set into motion.


Not only is the upward spread of masking considered to be one of the reasons why exposure to noise, which has been classed as a predominantly low frequency event, results in so many high frequency hearing losses. It is also one reason why low frequency sounds can cover over high frequency sounds, the concept previously referred to by Killion as 'self-masking.'


• A common complaint of hearing aid users is that their own voices sound too loud (Dempsey, 1990).

• This is often assumed to be the result of a sensorineural loss in which the person literally did not hear themselves for a long period of time, and find this re-acquaintance with their own voice to be something of a 'rude awakening'.


However, since occlusion of the ear canal causes such dramatic increases in the sound pressure of the patient/client's own voice (as we have just described), this loudness growth may have less to do with a loss of reception of their own vocalizations over a long time period, than it does with the change in delivery of their currently modified vocalization reception.


• Recruitment occurs not only in the high frequencies, but in the low frequencies as well.

• The response of the human auditory system to low frequencies requires more sound pressure be present before actual audition occurs.

• Once a low frequency sound is heard, recruitment can occur at a faster rate than in any other frequency range (Humes, 1985).


Let’s closely view this next slide. There is a lot of information on it. For now let’s learn the sound pressures required for each frequency to be audible to the human auditory system.



The magnification of a patient/client's own voice through occluded bone conduction (BC), results in a strong potential for low frequency recruitment.


• The patient/client’s voice is received at the ear through both AC and BC.

• In air conduction (AC), vocalizations must travel further distance at much slower speeds, resulting in airborne speech sounds arriving milliseconds later than those traveling to the ear via bone conduction (BC).


Estimates of the difference in time of transmission, based on average distance traveled and velocity of sound in a given medium (Zemlin, 1988; Speaks, 1992), are that AC speech sounds arrive at the ear approximately 6 msec later than BC speech sounds.


• Minimal time interval resolution (MTIR), has been shown to occur at intervals as brief as 3-4 msecs (Muchnik et al, 1985).

• MTIR generally slows with increasing age (Muchnik et al, 1985).

• Fastest resolutions occur at higher intensity levels (Muchnik et al, 1985); and in high frequency ranges (Irwin et al, 1981).


Sensorineural hearing loss (SNHL) has been shown to decrease resolution times (Irwin et al, 1981).


However, the increased intensities involved in BC vocalizations while the ear is occluded, combined with the difference in transmission time between air and bone conducted sound could cause some of the complaints of "echo effects" reported by hearing instrument users.


• Research on the effects of air pressure on the ears has indicated that increased air pressure causes reduction of acuity for low frequency AC sounds, (Weaver & Lawrence, 1954).

• Reduction in otoacoustic emission amplitudes, (Naeve et al, 1992).

• Continued subjective reports of individual ability to discriminate fine differences in barometric pressure based on feelings of pressure changes in the ear (Vernon, 1992).


This research suggests that there is a group of individuals who will have difficulty in dealing with air pressure changes brought on by occlusion.


Occlusion of the ear canal can also cause an increase in the loudness of tinnitus, resulting from either of two situations:1. Reduction of the masking effects of

external sound input can cause the loudness of tinnitus to become exaggerated.

2. The increase in the loudness of tinnitus may be due to occluding the ear canal itself (Vernon, 1992).


• The cases where the introduction of occlusion causes a marked increase in the loudness of tinnitus are relatively rare.

• However, awareness of the role of occlusion in tinnitus management is essential to the potential alleviation of tinnitus through hearing instruments.


Traditional definitions of occlusion have sometimes suggested that occlusion is a single effect, most often associated with changes in BC thresholds (Silman & Silverman, 1991). While bone conduction is clearly an important part of physical occlusion, this discussion has attempted to clarify the fact that occlusion involves multiple effects which involve all auditory processing pathways.

Health & Medicine

HIS 240 - Occlusion Physiology and Earmold Insertion Effects